Import a .py file from blender folder
The addons path(s) is already known to blender, hence you can import from any addon within a scripts/addons folder, without the need to alter sys.path.

from io_anim_bvh import import_bvh, export_bvh 
from space_view3d_copy_attributes import rotcopy


whether the addon you are importing from is registered or not.

There is also the scripts/modules folder.  Packages located there can be imported directly (akin to site-packages)

import osmium as o #  pyosmium